Albert Barnes Commentary 2 Samuel 15:18

"And all his servants passed on beside him; and all the Cherethites, and all the Pelethites, and all the Gittites, six hundred men that came after him from Gath, passed on before the king." — 2 Samuel 15:18 (ASV)

Passed on - This refers to crossing the Brook Kidron, as in 2 Samuel 15:22–23.

Gittites - During David’s residence in the country of the Philistines, he attached such a band to himself. After the settlement of his kingdom and the subjugation of the Philistines, the band received recruits from Gath, perhaps with the king of Gath’s consent. They were now under the command of Ittai the Gittite, a foreigner (2 Samuel 15:19), and “his brethren” (2 Samuel 15:20). The number 600 probably indicates that this band or regiment of Gittites had its origin in David’s band of 600 (1 Samuel 23:13; 1 Samuel 27:2). It is likely they were at first all Israelites, then Gittites mixed with Israelites, and finally all Gittites.